ARTIFICIAL PARTHENOGENESIS IN STARFISH EGGS - BEHAVIOR OF NUCLEI AND CHROMOSOMES RESULTING IN TETRAPLOIDY OF PARTHENOGENOTES PRODUCED BY THE SUPPRESSION OF POLAR BODY EXTRUSION

Artificial parthenogenesis in starfish requires the activation of oocy tes and the suppression of the polar body (PB) extrusion. To induce pa rthenogenesis we have employed a combination of calcium ionophore A231 87 for activating oocytes and cytochalasin B (CB) for the suppression of PB extrusions. The treatment of activated oocytes with CB during me iosis I produced eggs lacking polar bodies, i.e., Opb eggs, and treatm ent during meiosis II induced eggs bearing the first PB only, i.e., 1p b eggs. About 90% of both Opb and 1pb eggs developed parthenogenetical ly with strong synchrony of cell division among eggs. After meiotic ch romosomes in CB-treated oocytes separated finally into monads, they ca me together to form a single nucleus. The nucleus was found to be tetr aploid in the Opb eggs and diploid in the 1pb eggs. Regardless of the difference in ploidy, both Opb and 1pb eggs developed as tetraploid em bryos. Observations of the behavior of meiotic chromosomes and nuclei revealed that in the Opb eggs, the first round of chromosomal replicat ion was followed by the first cleavage, as in normally fertilized eggs . In the 1pb eggs, on the other hand, two rounds of chromosomal replic ation were found to precede the first cleavage, indicating the occurre nce of one (first) round of chromosomal replication that is not accomp anied with cytokinesis. In the first round of mitosis, a bipolar spind le did not appear, but only a half spindle was formed, resulting in th e failure of both karyokinesis and cytokinesis. Thus, both types of pa rthenogenetic eggs became tetraploid before the first cleavage. At the first cleavage, both types of eggs formed a bipolar spindle and divid ed into a pair of blastomeres. Based on these observations, we suggest that the meiotic centrosomes remaining in these eggs by the failure o f PB extrusion are diverted into mitosis-organizing centers in the mit otic spindle, and this results in parthenogenetic development. (C) 199 4 Academic Press, Inc.